Spallation nuclides

To avoid interferences of isobaric atomic ions of different elements and polyatomic ions at the same nominal mass, off line separation of matrix elements or analyte separation can be applied and/or enrichment and hyphenated techniques such as HPLC- or CE-ICP-MS. Analytical procedures have been developed in the author s laboratory for the determination of spallation nuclides in an irradiated tantalum target using HPLC and CE coupled on line to ICP-MS after dissolution and separation of the tantalum matrix.12,17 The mass spectra of rare earth elements (REEs)... 

Table 9.40 Concentration of different spallation nuclides of the lanthanides ((j-g g 1) in an irradiated tantalum target.

This method is used mainly for short-lived radioactive nuclides produced by cosmic ray spallation, such as °Be, A1, Si, C1, and Ar (Table 5-1). Because these nuclides have relatively short half-lives, if there was any initial amount of the nuclides at the beginning of Earth history, the initial amount would have completely decayed away. The small amount that can be found in... 

In addition to cosmic ray spallation also produces many other radioactive nuclides. °Be is another example. Once cosmogenically produced, atoms of °Be are rapidly removed from the atmosphere by meteoric precipitation, and are absorbed onto surfaces of solid particles such as clay minerals. Hence, newly formed marine sediment contains some initial concentration of °Be. After removal from the atmosphere, the concentration of °Be in sediment decays away by p-decay to °B with a half-life of 1.51 million years (and a decay constant of 4.59 X 10 yr ). 

Energetic particles react with solid matter in a variety of ways. Low-energy particles in the solar wind ( 1 KeV/nucleon) are implanted into solids to depths of 50 nm. Energetic heavy particles penetrate more deeply and disrupt the crystal lattice, leaving behind tracks that can be imaged by or chemically etched and observed in an optical microscope. Particles with energies of several MeV or more may induce a nuclear reaction. The two main modes of production of cosmogenic nuclides are spallation reactions and neutron capture. 

Spallation occurs when a high-energy cosmic ray breaks a target nucleus into two or more pieces. These interactions commonly eject neutrons. The secondary neutrons slow down to thermal energies and eventually react with other nuclei in the target material to generate heavier species. Production of cosmogenic nuclides by secondary neutrons increases with depth to a peak at between 0.5 and 1 m below the surface. Therefore, in order to get an... 

Cosmic-ray exposure ages are determined from spallation-produced radioactive nuclides. Cosmic-ray irradiation normally occurs while a meteoroid is in space, but surface rocks unshielded by an atmosphere may also have cosmogenic nuclides. These measurements provide information on orbital lifetimes of meteorites and constrain orbital calculations. Terrestrial ages can be estimated from the relative abundances of radioactive cosmogenic nuclides with different half-lives as they decay from the equilibrium values established in space. These ages provide information on meteorite survival relative to weathering. 

Table 9.41 Nuclide abundances of gadolinium (%) produced via spallation reactions in an irradiated tantalum target via spallation reactions.

Here on Earth, spallation facilities are being built, not just to produce specific nuclides, but to provide a source of neutrons. Typically, a lead or mercury target... 

Note After Bierman (1994), Gosse and Phillips (2001), and Lai (1988). See Gosse and Phillips (2001) for details about radionuclide decay constants and production rate uncertainties. Isotopes in italics are those that have been used routinely in cosmogenic nuclide studies. Stable isotopes will resolve paleoaltimetry for Cenozoic and older rocks. High-latitude, sea-level rates from empirical studies only. Production mostly via neutron spallation unless noted. Recent debate on die half-life of 10Be has focused on whether this number is off by >10%. 

There are three principal types of nuclear reactions due to the interactions of terrestrial materials with cosmic rays (i) by high-energy spallation of nucleons (E > 40MeV), principally by neutrons, (ii) by thermal neutron capture, and (iii) muon-induced nuclear disintegrations. Muon reactions become important only at depths below sea level. The estimation of the production ratio is difficult because of lack of knowledge of the probabilities of formation of nuclides in the different reactions. 

The spallation reaction is another example of special nuclear reactions, by which many nuclides relatively small mass number (about 10 to 20, the smaller the number, the higher the yield) in comparison with the target nuclide is produced simultaneously. The example is ... 

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